1. Field of the Invention
The present invention relates in general to transducers for converting a force into an electrical signal and methods of manufacturing same, and in particular to force transducers wherein force transmitting apparatus applies a force related to an input force at two spaced points of application to a transducing element.
2. Description of the Prior Art
Force transducers of this type may be used, for example, for transducing and measuring a force transmitted from the load carrying member of a scale, such as for a metering or proportioning system, to the force input of the force transducer. The metering system may comprise, for example, a conveyor belt adapted to convey metered materials, and a conveyor type scale having a load carrying member adapted to have the conveyor belt move on it. However, the force transducer may also be used in conjunction with a so-called gravimetric metering system, in which the material to be metered is conveyed by a conveyor screw or worm, or other type of conveyor member, away from and/or out of a material storage device, and the weight loss of the storage device is then determined using a weighing scale to control the rate at which the material is conveyed.
One type of force transducer having a force transmitting apparatus and an elongated transducing element responsive to the transmitted force applied at two points of application is disclosed in published Japanese patent application Sho-61-194 325 of Nishiguchi, and in European patent application 0 318 152 and the corresponding U.S. Pat. No. 4,856,350 to Hanson. As disclosed in the Nishiguchi reference, the force transducer has a rhombic or elliptical frame having at two opposing rhomb corners/ellipse vertices, a reference member fixedly mounted on a base or the like, and a movable force input member to which the input force is applied. At the two other rhomb corners/ellipse vertices, holders are provided which are adapted to hold the transducing element on the frame The holders are connected slightly movably to the reference member and the force input member by the legs of the frame, which constitute transmitting links. The ends of the transmitting links are constituted, each constriction forming a bending articulation or joint. The transducing element consists of a force detection vibrator, the exemplary form of which comprises a double-tuning-fork-shaped vibrator having two elongated tines mutually connected at their two ends by a web, and two piezoelectric elements mounted on the side faces of on of the connecting webs at the bases of the tines. The frame, the holders and the vibrator are built as sections of a single-piece plate member. The Hanson references disclose the same rhombic form of transducer, which utilizes one or two elongated vibratable tines. The FIG. 6 and 8 embodiments are integrally formed from a single plate-like member. The FIG. 6 embodiment has the same tine construction as the Nishiguchi transducer and includes suitable electrodes mounted on the tines for vibrating the tines. In the FIG. 8 embodiment, the tines are connected at their ends by enlarged, generally triangular web members which are connected to the frame by short connecting links, and the mounting pads are elongated to extend over a majority of the length of the frame. The Nishiguchi force transducer has its plate member made of metal, while the FIG. 6 and 8 embodiments of the Hanson force transducer are made of quartz. Both the Nishiguchi and Hanson transducers are electrically connected with an electronic device.
This type of conventional transducer suffers from a number of disadvantages. When operating this type of force transducer, a pressing force is applied to the force input member, which pushes the force input member against the reference member, resulting in the transducing element holders being pushed away from each other and the holders exerting a tension force on the transducing element. However, the pressing force acting at the reference member and the force input member tends to generate bending and/or torsional deformations that will bend certain regions of the frame in an undefined manner, outwardly from the plane defined by the resting frame, and/or may even buckle, the frame, resulting in false measurements and possibly permanent damage to the transducer.
The Nishiguchi plate members are relatively thick, of the order of several millimeters, presumably to the undesirable bending and torsional deformations just described. The vibrator and other movable parts of the Nishiguchi transducer thus have comparatively large masses in relation to the length of the vibratable tines. However, movable parts having large masses are disadvantageous because during measurement they are very sensitive to external shocks and disturbances. Thus, a force transducer having such large mass movable parts which is exposed, for example, during transportation or in conjunction with work on a system with which the force transducer is used, to powerful external shocks or disturbances could easily become damaged. Furthermore, if the plate members are thick, it becomes difficult to produce narrow weblike sections, which are desirable, for example, for forming the bending joints and vibrator tines of the force transducer. According to the aforementioned Nishiguchi reference, both the force transmitting device and the vibrator are cut out of a solid metallic plate member by spark erosion. Contour cutting a plate member several millimeters thick by spark erosion is, however, difficult and time consuming.
According to FIGS. 6 and 8 of the aforementioned Hanson European patent disclosure, the maximum dimension of the illustrated plate members as measured parallel to the longitudinal direction of the vibrator is approximately 10 mm or 12 mm, respectively, and the thickness of the plate members is approximately 0.15 mm and 0.25 mm, respectively. It is very doubtful whether it would be possible with so dimensioned plate members to prevent undesirable bending and torsional deformations from occurring which cause sections of the plate members to move out of the rest state planes defined by the plate members. In addition, it is complicated and expensive to cut a force transmitting device and a vibrator having the shapes shown, out of a solid quartz crystal.
Further, in the Nishiguchi and Hanson force transducers, the piezo-electric element/electrodes applied to the vibrator and the electrical conductors connected with the electronic device can disturb the vibrations of the vibrator, and thus reduce measurement accuracy. In addition, the vibrators of these known force transducers are believed to be closely coupled vibrationally with the force transmitting device, with the effect that during measurement the vibrations of the vibrator will be dampened to a great extent, and the measurement accuracy reduced.
Another conventional force transducer is disclosed in U.S. Pat. No. 4,541,495 to Willi et al which comprises a frame stationary during measurement and serving as a reference member, and a two-armed spring-mounted lever pivotably supported on the frame to receive and transmit the force to be measured. That is, the lever serves as the force input member and force transmitting device. A transducing element consists of a metallic, electrically conducting string having nodal masses and connected at one end with the frame by a holder, and at its other end with the lever by another holder. Each holder comprises a locking screw and a clamping jaw fastened to the frame and the lever, respectively. Further, the frame has a magnet fastened thereon which is adapted to generate a magnetic field in the central region of the string. The ends of the string extend past the holders and are connected with an electronic device adapted to supply a current through the string during measurement. If the frame and the lever are made of metal, the holders would also need to be provided with insulators to electrically insulate the string ends with respect to the frame and the lever. The Willi et al force transducer has the disadvantage of comprising a relatively large number of components having many different configurations. The manufacture and assembly of such a force transducer is thus very elaborate. Furthermore, the Willi et al force transducer has the string holders fastened only at one of their sides on the frame and at the end of one of the arms of the lever, respectively. Since the lever must transmit a force to the string, and since this string is subjected to tension stress, the frame, the lever and the holders must have comparatively large bending moments. The frame, the lever and the holders must therefore have high strength and must have, in comparison to the length of the string, large dimensions, thus increasing the space requirements of the transducer. The lever and the holder mounted on the lever thus also have large dimensions. As noted above, movable parts with large masses are undesirable.